The design of cellular systems has evolved to pursue the spatial capacity from simply pursuing the spectrum efficiency. Cell splitting is a common method to improve the spatial reuse efficiency of the spectrum. However, the cell radius in the fourth generation mobile communication (4G) system has been relatively small, and the potentiality for continuous splitting is poor. In addition, the distribution of users in space is not uniform, and a large number of users in a cell may locate in some hot spots and occasions. A heterogeneous network is more suitable for this scenario of uneven user distribution. The basic ideas of the heterogeneous network are as follows. A macro base station transmitting with larger power seamlessly covers a macro cell, and provides basic access; low-power base stations are deployed in hot occasions of the macro cell and provide high-speed data services. Low-power base stations may include pico base stations, femto base stations, and relay nodes (hereinafter uniformly referred to as micro base stations).
In a homogeneous network, inter-cell interference is serious only at the edge of a cell. However, in a heterogeneous network, the interference between the macro cell and an inside micro cell has unique property. The deployment of a low-power node is more random. For instance, an indoor femto base station is deployed by the user, and the location and switch thereof are random. The femto base station may be located at the edge of a macro cell, or may be located in the center of the macro cell. The power of a macro base station differs considerably from that of a micro base station, which results in asymmetric downlink.
Inter-cell interference mitigation techniques are of the following three main types. The first one is avoidance or orthogonal division through time, frequency, and spatial domain, such as the Fractional Frequency Reuse (FFR) technique and the Almost Blank Subframes (ABSF) technique. The second one is to perform power control with base station coordination so as to decrease the interference level, thus the interference may be taken as background noise, such as the Soft Frequency Reuse (SFR) technique. The third one is interference cancellation. When the interference is strong, the interference can be demodulated and then canceled.